In: Anatomy and Physiology
Describe the histology of the respiratory tract and how that changes between the trachea and the alveoli.
The respiratory tract consists of nasal cavities, nasopharynx, larynx, trachea, bronchii and bronchioles.
The trachea branches to give rise to two primary bronchii. These then branch successively to give rise in turn to secondary and tertiary bronchii. These then branch to give rise to several orders of progressively smaller airways called bronchioles, the smallest of which are called terminal bronchioles. These are the last components of the conducting portion of the respiratory system. Terminal bronchioles give rise to respiratory bronchioles, which ultimately lead to the alveoli.
Histology of trachea
The respiratory mucosa is made up of the epithelium and supporting lamina propria. The epithelium is tall columnar pseudostratified with cilia and goblet cells. The supporting lamina propria underneath the epithelium contains elastin.The sub-mucosa contains glands which are mixed sero-mucous glands. The watery secretions from the serous glands humidify the inspired air. The mucous, together with mucous from the goblet cells traps particles from the air which are transported upwards towards the pharynx by the cilia on the epithlium.
Histology of bronchi
The trachea branches into two primary bronchii, which branch into secondary and then tertiary bronchii. In the tertiary bronchii, there is less cartilage, and it does not completely encircle the lumen.
Histology of bronchioles
The epithelium is made up of ciliated columnar cells in larger bronchioles, or non-ciliated in smaller bronchioles. There are no goblet cells, but there are cells called Clara cells. These cells are secretory. They secrete one of the components of surfactant.
Histology of terminal bronchioles
These have a layer smooth muscle surrounding their lumens.Small sacs are found extending from the walls of the terminal bronchii called respiratory bronchioles, that are lined by a ciliated cuboidal epithelium, and some non-ciliated cells called clara cells.
Histology of alveoli
The alveoli are lined by flattened epithelial cells called pneumocytes with a single opening. The alveolar wall or septum is made up of three tissue components: surface epithelium, supporting tissue, and capillaries. Capillaries are surrounded by a vibrant network of elastin, reticular and collagen fibers with a layer of squamous epithelial of two adjacent alveoli on either side. They contain 2 types of cells:-
Type I Pneumocytes (Alveolar lining cells)
Type II Pneumocytes (Great alveolar or septal cells)